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(SBUs). FJU-22 has a unique type of helical chain SBU with dif-
ferent pitches composed of the PWNs and N-benzene triazole
linkers along the a and c axes (Figure 1j and k). The linkage
between adjacent SBUs sharing the PWNs results in a three-di-
mensional (3D) skeleton (Figure 1l and m). FJU-21 has two
types of helical chain SBUs along the b axis. One has the
same composites as that in FJU-22 (Figure 1c), which connects
with the neighboring SBUs to give triazole-pillared
[Cu2(isophthalate)4] bilayers in the orientation of the (200)
plane (Figure 1e), whereas the other is made up of the PWNs
and isophthalate linkers (Figure 1d), and connects with the ad-
jacent SBUs to produce [Cu2(isophthalate)4] monolayers orient-
ed at the (100) plane (Figure 1 f). The bilayers and monolayers
stack on each other by layer-sharing along the a axis to form
the 3D framework (Figure 1g). By considering the PWN as
a six-connected octahedral node and the ligand as a three-con-
nected trigonal linker, the whole frameworks of FJU-21 and
FJU-22 can be simplified to a (3,6)-connected net with rutile
(rtl) and a-PbO2 (apo) topology, respectively. FJU-21 shows
a 1D channel along the a axis (5.009.60 2), whereas FJU-22
also has a 1D channel, but along the c axis (7.107.10 2; Fig-
ure S1 in the Supporting Information). PLATON calculations[34,35]
Gas adsorption
To assess the permanent porosity, the N2 sorption isotherms of
the activated FJU-21a and FJU-22a materials were examined
at 77 K (Figure 2), which yielded a reversible type I isotherm for
the microporous nature of the samples with Brunauer–
Emmett–Teller (BET) surface areas of 369.10 and 828.19 m2gÀ1,
respectively. FJU-21a shows a bimodal pore size distribution
centered at 5.2 and 8.7 , and FJU-22a has a distribution cen-
tered at 8.0 , as calculated by the non-local (NL)-DFT method;
these values are close to the pore sizes determined from the
crystal structures (Figure S1 in the Supporting Information). Al-
though their void volumes from the Platon calculations are
close, the BET surface area for FJU-21a, with the dynamic
framework, is only about half that for FJU-22a. The flexible
character of FJU-21 is further confirmed by a hysteresis loop in
the N2 adsorption isotherm at 77 K.
The unique pore structures encouraged us to examine the
capacities of the two MOFs for gas adsorption. The low-pres-
sure sorption isotherms of CO2, C2H2, and C2H4 were collected
at 273 and 296 K (Figure 2 and Figure S3 in the Supporting In-
formation). At 296 K and 1 bar, FJU-22a can adsorb 111.3,
114.8, and 85.8 cm3gÀ1 of CO2, C2H2, and C2H4, respectively. The
adsorption isotherms for C2H2, CO2, and C2H4 on FJU-21a are
very similar to those for FJU-22a and the adsorption capacity
follows the same hierarchy: C2H2 >CO2 >C2H4. This phenomen-
on may be attributed to the same pore surface structure re-
sulting from the same metal node and ligand connection
mode. However, the halved BET surface area for FJU-21a
makes its various gas uptakes fall to half the corresponding
values of FJU-22a. Furthermore, it is worth noting that the
acetylene uptake isotherms for FJU-21a and FJU-22a at 296 K
show a very sharp uptake at low pressure, whereas carbon di-
oxide uptake is much lower at this pressure. This discovery
motivated us to examine their feasibility for the industrially im-
portant C2H2/CO2 separation.
3
of FJU-21 and FJU-22 indicate their void volumes are 923.7
3
(52.1% of the unit cell volume of 1773.3 3) and 1908
(52.8% of the unit cell volume of 3614.2 3), respectively.
Owing to the same metal node and metal–ligand connec-
tion mode, FJU-21 and FJU-22 have similar pore surface struc-
tures. Nevertheless, it is worth noting that the solvent-induced
structural diversity gives the two MOFs distinct robustness
properties. The two as-synthesized MOFs were exchanged with
CH3OH and CH2Cl2, respectively, several times, then heated to
608C, and evacuated under high vacuum to obtain the desol-
vated frameworks FJU-21a and FJU-22a. FJU-21a is flexible,
whereas FJU-22a shows good robustness, as proved by
powder X-ray diffraction experiments (PXRD; Figure S2 in the
Supporting Information). For FJU-22, with the one unique type
of helical chain SBU, the 2q values are not shifted for the acti-
vated sample compared to the as-synthesized sample pattern.
However, for FJU-21, which contains one more type of helical
chain SBU, the values of 2q for the (100) and (002) planes are
shifted to higher angles for the activated sample, and no shift
for the (020) plane is seen, indicating that the dynamic fea-
tures are down to the [Cu2(isophthalate)4] monolayers oriented
at the (100) plane and constructed from the helical chains ex-
clusively in FJU-21 and not observed in FJU-22. In addition, if
exposed to air or water vapor, the values of 2q for FJU-21 are
shifted, whereas for FJU-22 there is no obvious change under
the same conditions, further indicating that FJU-22 has better
stability than FJU-21. Although several methods including
those using high-valent metal ions,[36] modulated synthesis,[37]
N-donor ligands,[38] and superhydrophobic ligands[39] have
been proposed to enhance MOF stability, FJU-21 and FJU-22
are the first examples to demonstrate control of MOF stability
and robustness by adjusting the helical chain SBUs.
C2H2/CO2 column breakthrough experiments
We first performed breakthrough simulations for a 50:50 (v/v)
C2H2/CO2 mixture on FJU-21a and FJU-22a by using the estab-
lished methodology.[40] As shown in Figure S4 (in the Support-
ing Information), FJU-21a and FJU-22a are able to separate
C2H2 from the C2H2/CO2 mixture at room temperature. Clearly,
FJU-22a, with good robustness, is more effective than FJU-
21a for the C2H2/CO2 separation. Thus, we only studied the
actual performance of FJU-22a in the experimental column
breakthrough.
In the actual column breakthrough experiment, an equimo-
lar C2H2/CO2 mixture was flowed over a packed column of the
FJU-22a solid with a total flow of 5 cm3 minÀ1 at 296 K
(Figure 3). CO2 was detected after the gas mixture has been in-
troduced into the column for about 12 min, whereas C2H2 was
not detected until a breakthrough time of 23 min was reached.
Thus, the separation of C2H2/CO2 mixture gases through
a column packed with FJU-22a solid can be achieved efficient-
ly. The breakthrough times of CO2 and C2H2 on the unique
Chem. Eur. J. 2016, 22, 5676 – 5683
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